Coal as a fuel is an abundant resource of energy comprising mostly carbon, and small percentages of hydrogen, sulfur and ash. When coal is burned to produce energy, the presence of the sulfur and ash is generally undesirable. The ash enters the atmosphere as small particles (particulates) and the sulfur as noxious sulfur oxide gases. Sulfur is present in coal in three principal forms: pyritic sulfur (a combination of iron and sulfur); sulfate sulfur, generally in very small quantities, say 0.5 to 0.1 percent by weight; and organic sulfur, that is chemically combined sulfur within the coal structure.
Pyritic sulfur can, to a large extent, be washed out of coal by conventional coal washing methods. These methods are not, however, suitably efficient on a large scale and at best only a small portion of the mined coal can be sufficiently up-graded by washing alone.
Sulfate sulfur can easily be separated from coal by dissolving it in water. For example, it may boiled out of the coal matrix by elevated temperature processes which have already been developed.
At the present time there appears to be no commercial process for removing organic sulfur from coal. Removal of organic sulfur requires drastic chemical treatment causing the breaking of bonds between the sulfur and the carbon within the structure of the coal molecule. Where the sulfur content of coal is very near the permissible level as designated by government anti-pollution laws and regulations, it still may not be possible to economically upgrade the coal by removal of organic sulfur. Thus, it becomes necessary to treat exhaust gases with expensive scrubbers which use large quantities of chemicals and which can create additional pollution problems.
Processes have been conceived and to some extent developed for removal of a portion of the organic sulfur coal. At this time, they require very expensive treatment facilities utilizing high pressures say up to 500 to 1,000 psi, and temperatures up to 600.degree. F. (about 400.degree. C.). Clearly, from an engineering and processing point of view, it does not make sense to treat coal in order to reduce the initial sulfur content of the coal from say 1.5% to a 0.6 to 0.8% level by use of these processes.
Summarizing the numerous processes which have been proposed for upgrading coal to remove various forms of sulfur, the following have been considered: (1) Oxidation of sulfur in the coal in an aqueous medium to form soluble sulfates; (2) reduction of the sulfur to elemental sulfur in which form it can be vaporized or removed by organic solvents; (3) reaction with hydrogen to form gaseous hydrogen sulfide; (4) vapor deposition selectively on the pyritic form of sulfur followed by magnetic separation of the pyrites; (5) oxidation of the sulfur with nitric oxide vapors to form gaseous sulfur oxides; (6) leaching with a sodium and calcium oxide lixiviant; and (7) leaching with aqueous ferric sulfate.
The applicant's process disclosed herein has the potential for providing a commercial process for removal of the three basic forms of sulfur from coal and coal-like materials. At the same time, the process reduces the amount of ash within the coal or coal-like material. The process involves the use of atmospheric pressures and low temperatures (temperatures near room temperature) and may be practiced with rugged processing equipment.